1. Field of the Invention
The present invention relates to a photographic coupler and, more particularly, to a novel 2-equivalent magenta coupler for use in a silver halide color photographic light-sensitive material.
2. Description of the Prior Art
It is known that, upon color development of a silver halide color photographic material, an oxidized aromatic primary amine color developing agent reacts with a coupler to form an indophenol, indoaniline, indamine, azomethine, phenoxazine, phenazine or like dye, with color images being formed. In this system, color reproduction is usually based on subtractive color photography, and silver halide emulsions selectively sensitive to blue, green and red light, respectively, and yellow, magenta and cyan color image-forming compounds which are in a complementary color relation to the sensitivity of the respective emulsions are employed. For example, acylacetanilide or dibenzoylmethane couplers are used for the formation of a yellow color image, pyrazolone, pyrazolobenzimidazole, cyanoacetophenone and indazolone couplers are mainly used for the formation of magenta color images, and phenolic couplers (e.g., phenols and naphthols) are mainly used for the formation of cyan color images.
In one of the most preferred embodiments of color photographic light-sensitive materials, dye image-forming couplers are added to silver halide emulsions. Couplers added to emulsions must be rendered non-diffusible (or diffusion-resistant.)
Almost all conventional color image-forming couplers are 4-equivalent couplers. That is, the development of 4 moles of silver halide as an oxidizing agent is theoretically necessary to form 1 mol of dye through the coupling reaction. On the other hand, 2-equivalent couplers having an active methylene group substituted with a group eliminatable upon the oxidative coupling with an oxidation product of an aromatic primary amine developing agent requires the development of only 2 moles of silver halide to form 1 mol of dye. Since 2-equivalent couplers require only one-half the silver halide as compared with ordinary 4-equivalent couplers to form a dye, their use enables the rapid processing of light-sensitive materials due to the thinness of the light-sensitive layers, the photographic properties to be improved due to a reduction in film thickness, and economic advantages to be achieved.
Several approaches have thus far been suggested to produce 2-equivalent 5-pyrazolone couplers mainly used as magenta-forming couplers. For example, the substitution of the 4-position of a pyrazolone with a thiocyano group is described in U.S. Pat. Nos. 3,214,437 and 3,253,924, with an acyloxy group is described in U.S. Pat. NO. 3,311,476, with an aryloxy group is described in U.S. Pat. No. 3,419,391, with a 2-triazolyl group is described in U.S. Pat. No. 3,617,291, and with a halogen atom is described in U.S. Pat. No. 3,522,052.
However, in using these 4-position substituted pyrazolone couplers, the disadvantages that serious color fog results, that the reactivity of the couplers is unsuitable, that the couplers are chemically so unstable that they are converted to materials incapable of color formation with the lapse of time, so that synthesis of the couplers often is difficult occur.
Also, it has hitherto been known to substitute the 4-position of a 5-pyrazolone with an alkylthio group, an arylthio group or a heterocyclic ring thio group as described in U.S. Pat. No. 3,227,554. However, with many of these known thio-substituted pyrazolone compounds, the reactivity with an oxidation product of an aromatic primary amino color developing agent is unsuitable and, further, they are difficult to employ in ordinary color light-sensitive materials due to the strong photographic action of the mercapto compound produced as a result of the coupling reaction. In addition, the chemical stability of these couplers is not satisfactory.